Tracking second object to a pose
OK, for anybody interested, I figured out what I was doing wrong.  Clearly a newbie mistake.
My mistake was to try to make the arrow mesh the child of the body mesh.  Instead, I needed to make the arrow mesh the child of an individual bone in the armature. The trick was to choose the right bone, since in my case the spine had a few of them. Nonetheless, once I chose the right bone, it worked great.
The other thing that misled me a little was that my human is an export from Makehuman, and the naming of the parts was a little confusing.  When I first tried to add constraints I couldn't find the armature to try it, but on review, it was there.  In my case Export: was the mesh, and &quot;Export&quot; itself was the armatureds and the human in general.
So, here you see me making the arrow a child of the left clavicle of the &quot;Exports&quot; armature (and also note that everything seems to be labeled &quot;Exports&quot;).  Also note that I did not include Scale in the parenting.

Now I go into pose mode with the human, and move any of the bones, and the arrow follows the clavicle like it should:

So, it turns out that it was easy (once I spent a few hours working on it...)